Lattice instabilities and critical fluctuations in high-Tc superconductors

The double well potential of the pyramidal oxygen atoms in the high-Tc superconductors is studied, below Tc, in the region of critical fluctuations. The order parameter critical fluctuations are treated in the Gaussian approximation of the Ginzburg-Landau theory. By applying path integral techniques an expression for the tunneling frequency between the minima of the double well potential is derived. It is shown that non-BCS temperature-dependent gap functions account for the large enhancement of the tunneling frequency observed at the onset of superconductivity. The lattice instability due to the apex oxygen atoms can be responsible for the anomalous specific heat in the critical region below Tc. © 1992.